Electronic variable gain amplifiers (VGA) that provide a decibel linear gain response in relation to a gain control signal are known. Such a VGA has an input for receiving a signal and an output for providing the input signal amplified by the gain of the VGA which is determined by a gain control signal.
In operation, one requirement of the VGA is that its gain response in decibels be linear with respect to the level of the gain control signal. While this can be accomplished when the VGA is operating at relatively low gain, when a higher gain is required the gain of the VGA in decibels can become non-linear in relation to the level of the gain control signal. This occurs because beyond a certain gain, the gain of the amplifier begins to reduce or roll off. Another requirement of the VGA is that its gain response in decibels versus the level of the gain control signal be less sensitive to variations in temperature.
One method of obtaining a linear relationship between the gain in decibel and the gain control signal was disclosed in U.S. Pat. No. 4,816,772 by Klotz where feedback is employed to generate a conditioned control voltage.
A disadvantage of this method is that there is no effective temperature compensation. Another disadvantage is the need for good matching between the stages to avoid discontinuities in the gain versus gain control signal characteristic of the VGA.